Braided mechanical packing of a generally square or rectangular cross section has been utilized for many years to seal rotary shafts to the adjacent stuffing box to prevent or minimize the leakage of fluid. The leakage is from the interior of the equipment sealed and runs along the shaft/packing interface to the atmosphere.
As illustrated in U.S. Pat. Nos. 4,550,639; 4,672,879; and 4,729,277 by George B. Champlin, incorporated herein by reference, there is a keystoning problem associated with the use of packing rings having square cross sections when straight. The problem is that the square cross section becomes trapezoidal when the packing is wrapped around the shaft. Note, when wrapped, the wide side of the trapezoid is towards the shaft. The result is scoring of the shaft when the packing is compressed in the stuffing box due to the inside corners of this packing being compacted at the shaft.
This keystone problem is alleviated through the utilization of a packing ring having a trapezoidal shape when straight. When the packing is installed around a shaft with the wider portion of the trapezoidal cross section away from the rotating shaft, the packing changes from its trapezoidal cross section to a square cross section, which shape it maintains when it is compressed in the stuffing box. Because the square cross section has been formed naturally during the wrapping process prior to compression within the stuffing box, and not by means of outside pressures forcing it into the square shape, the scoring and uneven wear of the shaft associated with conventionally braided packing is eliminated. Note, the trapezoidal shape of the packing is provided by braiding the packing on two, three, or four track braiders in which more axial warp fill is added to what will become the outer corners of the braided packing than is used in what will become the inner corners of the packing. The result is a generally trapezoidally-shaped packing which can be achieved without calendaring or passing the packing through a die.
In order to achieve the squaring of the two convergent sides of the trapezoidally-shaped packing when wrapped around the shaft, after several years of experiments, braiding formulas were adjusted such that squaring of the sides would occur about the smallest shaft size commonly used with a given size of packing. For example, with 5/8 inch size packing, the optimal shaft about which this packing can be wrapped is about 3-1/2 inches in diameter. When such a trapezoidally shaped packing is wrapped about this optimal diameter shaft, the sides of the packing square up from the trapezoidal shaped cross section to a square cross section.
However, when the equipment to be sealed has larger diameter shafts, and such equipment exists with shaft diameters up to 39 inches as an example, the 5/8 inch packing is not bent to as small a radius as the optimal shaft since the shaft about which it is wound has a larger diameter. This means that the converging sides of the packing do not exactly square up. Rather, the wrapped packing retains a somewhat trapezoidal cross section, with the sides of any given ring converging towards the shaft at a slight angle. If the packing is not installed properly, the result is packing rings that shift out of their installed position spin during break-in, which results in uncontrollable leakage.
In summary, while providing trapezoidally-shaped packing is a practical solution to the keystone problem because it works to prevent uneven shaft wear for improperly installed packing, there is nonetheless a problem of packing ring spin during break-in. Break-in is a procedure which refers to the process of tightening the packing gland when the machine into which the packing is installed is first turned on. If improperly installed, these packing rings can spin and the joints formed at the ends of a packing ring can open up. The result of packing ring spin and joint opening is uncontrollable leakage.
Were this packing always installed in the stuffing box in the manner specified by the manufacturer, namely one packing ring at a time, with the first ring being pushed against the bottom of the stuffing box, and the second and subsequent rings being pushed into contact with the previous ring and/or a lantern ring, then regardless of shaft size the rings would not spin, but rather would stay in the position they were placed when installed.
However, in practice, despite instruction both written and verbal to the mechanics who install the packing, there is a significant percentage who insist upon retaining their lifelong habit of installing the packing as they were originally taught, which is to push the first ring into the stuffing box using the second ring, and to push the first and second rings deeper into the stuffing box with the third ring, and so forth until the stuffing box has the requisite number of packing rings. Because of resistance from the friction of the rings already in the stuffing box, when an outer ring pushes against the adjacent inner ring, the rings, instead of being pushed into the stuffing box all the way get thicker in the radial direction, which causes the inner and outer surfaces to bind against the shaft and the stuffing box bore. The result is that the ring nearest the bottom of the stuffing box never seats. Analysis of rings installed as a group show that while the rings closest to the gland are square, those towards the bottom of the stuffing box retain some trapezoidal shape. This indicates that the drawing force of the gland axially down the shaft is dissipated in which the force becomes radial as opposed to longitudinal. Thus the force down the shaft which would push the rings to the bottom of the stuffing box is diverted outwardly. This being the case, the bottom ring never gets pushed to the bottom of the stuffing box with enough force to seat it. During startup, also known as break-in, if the rings are not seated, the rings will spin and joints will open up.
The joints open up because when the ring starts to spin, the end of the ring against which the rotational force pushes, is pushed in the direction of rotation, which opens up a gap between the butted ring ends.
Thus, when improperly installed, the outer portion of the first ring does not actually touch the bottom of the stuffing box, and if it does, there is insufficient pressure on the sides of the inner rings to retain them in their installed position when the equipment is started. Consequently, there is also insufficient frictional contact between the rings and also insufficient pressure on the inner rings to keep them from spinning.
There is also another problem with traditional packing installation, and that is the almost universal use of grease. To help ease the rings into the stuffing box, mechanics routinely apply a coating of grease or other lubricant to all surfaces of the packing rings, a practice passed down through time when one of the principle ingredients of packing was grease or tallow. These types of packing and the practice of adding the coating of grease or tallow were originally used with equipment having reciprocating rods rather than rotating shafts.
The application of grease or other lubricants was and is still used to reduce the friction when pushing the rings down the bore of the stuffing box in the traditional method. However, the use of grease adds to the problem of packing ring spin by reducing the friction between the bore of the stuffing box and the packing rings and between the rings themselves. Note, that it is this necessary friction that holds the packing rings stationary at startup.
Thus, when the mechanic installs trapezoidally shaped packing in the traditional manner, he greatly reduces the service life of the packing he is installing by inadvertently helping to create the mode by which the packing rings are more likely to spin or have their joints at the ends of the rings separate at startup.
Further, much packing is installed by mechanics who do not receive the manufacturers installation instructions due to the practice of issuing a length of packing cut from a bulk roll. Thus the failure to communicate with all who use the modern packing makes it impossible to insure that the packing is properly installed according to modern practice.
As mentioned before, the effect of the rings spinning is twofold. First, there is a leak path formed between the packing and the stuffing box bore and/or the opened joint between the ends of the ring, which creates excessive leakage of fluid from the stuffing box. Second, the spinning ring creates undesired wear at the bore of the stuffing box causing the need for its unnecessary and premature replacement, since wear is planned to be only at the packing/shaft-sleeve interface.
Moreover, the joints formed by the abutting ends of a ring may also open due to the spinning, with the length of the rings shortening to create a gap between the ends of the ring. The open joint thus becomes the source of uncontrollable leakage regardless of how much gland pressure is applied, since a discontinuity or unfillable void has been created in the packing set. This excessive, uncontrollable leakage also causes the need to repack the equipment prematurely.
As a result of these practices, much of the packing used does not provide the maximum length of service time it is capable of providing, raising the costs of packing, installation labor, and equipment downtime. Because of these practical problems there is a need to prevent the spinning or shifting of the packing rings in the stuffing box during the break-in procedure.